/* ntpclient.c */
#include <stdio.h>
#include <stdlib.h>
#include <stdint.h>
#include <stddef.h>
#include <string.h>
#include <unistd.h>
#include <endian.h>
#include <time.h>
#include <netdb.h>
#include <arpa/inet.h>
#include <netinet/in.h>
#include <sys/time.h>
#include <sys/select.h>
#include <sys/socket.h>

#define VERSION_3           3
#define VERSION_4           4

#define MODE_CLIENT         3
#define MODE_SERVER         4

#define NTP_LI              0
#define NTP_VN              VERSION_3
#define NTP_MODE            MODE_CLIENT
#define NTP_STRATUM         0
#define NTP_POLL            4
#define NTP_PRECISION       -6

#define NTP_HLEN            48

#define NTP_PORT            123
#define NTP_HOST            "ntp.aliyun.com"

#define TIMEOUT             10

#define BUFSIZE             1500

#define JAN_1970            0x83aa7e80

#define NTP_CONV_FRAC32(x)  (uint64_t) ((x) * ((uint64_t)1<<32))
#define NTP_REVE_FRAC32(x)  ((double) ((double) (x) / ((uint64_t)1<<32)))

#define NTP_CONV_FRAC16(x)  (uint32_t) ((x) * ((uint32_t)1<<16))
#define NTP_REVE_FRAC16(x)  ((double)((double) (x) / ((uint32_t)1<<16)))

#define USEC2FRAC(x)        ((uint32_t) NTP_CONV_FRAC32( (x) / 1000000.0 ))
#define FRAC2USEC(x)        ((uint32_t) NTP_REVE_FRAC32( (x) * 1000000.0 ))

#define NTP_LFIXED2DOUBLE(x)    ((double) ( ntohl(((struct l_fixedpt *) (x))->intpart) - JAN_1970 + FRAC2USEC(ntohl(((struct l_fixedpt *) (x))->fracpart)) / 1000000.0 ))

struct s_fixedpt {
    uint16_t intpart;
    uint16_t fracpart;
};

struct l_fixedpt {
    uint32_t intpart;
    uint32_t fracpart;
};

struct ntphdr {
#if __BYTE_ORDER == __BID_ENDIAN
    unsigned int ntp_li :2;
    unsigned int ntp_vn :3;
    unsigned int ntp_mode :3;
#endif
#if __BYTE_ORDER == __LITTLE_ENDIAN
    unsigned int ntp_mode: 3;
    unsigned int ntp_vn: 3;
    unsigned int ntp_li: 2;
#endif
    uint8_t ntp_stratum;
    uint8_t ntp_poll;
    int8_t ntp_precision;
    struct s_fixedpt ntp_rtdelay;
    struct s_fixedpt ntp_rtdispersion;
    uint32_t ntp_refid;
    struct l_fixedpt ntp_refts;
    struct l_fixedpt ntp_orits;
    struct l_fixedpt ntp_recvts;
    struct l_fixedpt ntp_transts;
};

in_addr_t inet_host(const char *host) {
    in_addr_t saddr;
    struct hostent *hostent;

    if ((saddr = inet_addr(host)) == INADDR_NONE) {
        if ((hostent = gethostbyname(host)) == NULL)
            return INADDR_NONE;

        memmove(&saddr, hostent->h_addr, hostent->h_length);
    }

    return saddr;
}

int get_ntp_packet(void *buf, size_t *size) {
    struct ntphdr *ntp;
    struct timeval tv;

    if (!size || *size < NTP_HLEN)
        return -1;

    memset(buf, 0, *size);

    ntp = (struct ntphdr *) buf;
    ntp->ntp_li = NTP_LI;
    ntp->ntp_vn = NTP_VN;
    ntp->ntp_mode = NTP_MODE;
    ntp->ntp_stratum = NTP_STRATUM;
    ntp->ntp_poll = NTP_POLL;
    ntp->ntp_precision = NTP_PRECISION;

    gettimeofday(&tv, NULL);
    ntp->ntp_transts.intpart = htonl(tv.tv_sec + JAN_1970);
    ntp->ntp_transts.fracpart = htonl(USEC2FRAC(tv.tv_usec));

    *size = NTP_HLEN;

    return 0;
}

void print_ntp(struct ntphdr *ntp) {
    time_t time;

    printf("LI:\t%d\n", ntp->ntp_li);
    printf("VN:\t%d\n", ntp->ntp_vn);
    printf("Mode:\t%d\n", ntp->ntp_mode);
    printf("Poll:\t%d\n", ntp->ntp_poll);
    printf("Stratum:\t%d\n", ntp->ntp_stratum);
    printf("Precision:\t%d\n", ntp->ntp_precision);

    printf("Route delay:\t%lf\n",
           ntohs(ntp->ntp_rtdelay.intpart) + NTP_REVE_FRAC16(ntohs(ntp->ntp_rtdelay.fracpart)));
    printf("Route disper:\t%lf\n",
           ntohs(ntp->ntp_rtdispersion.intpart) + NTP_REVE_FRAC16(ntohs(ntp->ntp_rtdispersion.fracpart)));
    printf("Referencd ID:\t%d\n", ntohl(ntp->ntp_refid));

    time = ntohl(ntp->ntp_refts.intpart) - JAN_1970;
    printf("Reference:\t%d\t%d\t%s",
           ntohl(ntp->ntp_refts.intpart) - JAN_1970,
           FRAC2USEC(ntohl(ntp->ntp_refts.fracpart)),
           ctime(&time));

    time = ntohl(ntp->ntp_orits.intpart) - JAN_1970;
    printf("Originate:\t%d\t%d\t%s",
           ntohl(ntp->ntp_orits.intpart) - JAN_1970,
           FRAC2USEC(ntohl(ntp->ntp_orits.fracpart)),
           ctime(&time));

    time = ntohl(ntp->ntp_recvts.intpart) - JAN_1970;
    printf("Receive:\t%d\t%d\t%s",
           ntohl(ntp->ntp_recvts.intpart) - JAN_1970,
           FRAC2USEC(ntohl(ntp->ntp_recvts.fracpart)),
           ctime(&time));

    time = ntohl(ntp->ntp_transts.intpart) - JAN_1970;
    printf("Transmit:\t%d\t%d\t%s",
           ntohl(ntp->ntp_transts.intpart) - JAN_1970,
           FRAC2USEC(ntohl(ntp->ntp_transts.fracpart)),
           ctime(&time));
}

double get_rrt(const struct ntphdr *ntp, const struct timeval *recvtv) {
    double t1, t2, t3, t4;

    t1 = NTP_LFIXED2DOUBLE(&ntp->ntp_orits);
    t2 = NTP_LFIXED2DOUBLE(&ntp->ntp_recvts);
    t3 = NTP_LFIXED2DOUBLE(&ntp->ntp_transts);
    t4 = recvtv->tv_sec + recvtv->tv_usec / 1000000.0;

    return (t4 - t1) - (t3 - t2);
}

double get_offset(const struct ntphdr *ntp, const struct timeval *recvtv) {
    double t1, t2, t3, t4;

    t1 = NTP_LFIXED2DOUBLE(&ntp->ntp_orits);
    t2 = NTP_LFIXED2DOUBLE(&ntp->ntp_recvts);
    t3 = NTP_LFIXED2DOUBLE(&ntp->ntp_transts);
    t4 = recvtv->tv_sec + recvtv->tv_usec / 1000000.0;

    return ((t2 - t1) + (t3 - t4)) / 2;
}

int ntp_sync(const char *host) {
    int rc = -1;
    int sockfd = 0, maxfd;
    size_t nbytes;
    fd_set readfds;
    struct timeval timeout, recvtv, tv;
    double offset;
    struct sockaddr_in servaddr;
    char buf[BUFSIZE];

    servaddr.sin_family = AF_INET;
    servaddr.sin_port = htons(NTP_PORT);
    servaddr.sin_addr.s_addr = inet_host(host ? host : NTP_HOST);

    if ((sockfd = socket(AF_INET, SOCK_DGRAM, 0)) < 0) {
        perror("socket");
        goto err;
    }

    if (connect(sockfd, (struct sockaddr *) &servaddr, sizeof(struct sockaddr)) != 0) {
        perror("connect");
        goto err;
    }

    nbytes = BUFSIZE;
    if (get_ntp_packet(buf, &nbytes) != 0) {
        fprintf(stderr, "construct ntp request error\n");
        goto err;
    }
    send(sockfd, buf, nbytes, 0);

    FD_ZERO(&readfds);
    FD_SET(sockfd, &readfds);
    maxfd = sockfd + 1;

    timeout.tv_sec = TIMEOUT;
    timeout.tv_usec = 0;

    if (select(maxfd, &readfds, NULL, NULL, &timeout) > 0) {
        if (FD_ISSET(sockfd, &readfds)) {
            if ((nbytes = recv(sockfd, buf, BUFSIZE, 0)) < 0) {
                perror("recv");
                goto err;
            }

            print_ntp((struct ntphdr *) buf);

            gettimeofday(&recvtv, NULL);
            offset = get_offset((struct ntphdr *) buf, &recvtv);

            gettimeofday(&tv, NULL);
            tv.tv_sec += (int) offset;
            tv.tv_usec += offset - (int) offset;

            if (settimeofday(&tv, NULL) != 0) {
                perror("settimeofday");
                goto err;
            }
            printf("settimeofday:\t%s\n", ctime(&tv.tv_sec));
        }
    }

    system("hwclock -w");

    rc = 0;
    err:
    close(sockfd);
    return rc;
}

int main(int argc, char *argv[]) {
    return ntp_sync(argc == 1 ? NULL : argv[1]);
}
